Pipetting devices are primarily used at laboratories to proportion liquid volumes. They have a moving device by means of which a piston is movable within a cylinder to displace a column of air. The cylinder is connected to the aperture of an attachment stem onto which a pipette tip can be detachably placed. The movement of the piston by means of the moving device permits to displace the column of air to suck a liquid sample into or expel from the pipette tip. The proportioning volume, i.e. the volume of the liquid sample, is defined by the stroke of the piston that is limited by a stop.
Adjustable pipetting devices have an adjustment device to shift the stop. The stop is frequently arranged within a spindle which is passed through by a piston rod of the piston. The piston rod has a counter-stop which interacts with the stop. The position of the stop and, thus, the piston stroke and the proportioning volume are adjustable by displacing the spindle in a spindle nut.
DE 25 49 477 C3 discloses an adjustable pipetting device of the aforementioned type in which the movement of the piston is limited by a stop joined to the spindle. If a passage is intended from a setting effected at one end of the range of adjustment over to a setting to be effected at the other end numerous rotations require to be made on the spindle or the setting knob joined to the upper end thereof. This needs a lot of time and is very troublesome.
EP 0 286 676 has made known a pipette with a rapidly variable pro-portioning volume. It has a casing in which a piston with a piston rod is movably arranged. The extent of piston stroke and, thus, the extent of proportioning is adjusted by a regulation bushing which is permanently coupled to a device for the visual read-out of the proportioning volume. This device serves for a rough adjustment. The position of the regulation bushing is set by rotating an adjusting nut which has a female thread meshing with the regulation bushing and can be rotated from the side of the casing. The adjusting nut carries an indication mark at the outer circumference to read its precise adjustment. The engagement of the adjusting nut with the thread of the regulation bushing can be cancelled by laterally forcing the adjusting nut inwardly against a resilient force. Then, the regulation bushing will be displaceable when a push-button of the piston rod is actuated, in which operation a transmission of forces takes lace via springs between the regulation bushing and the piston rod. The adjusting nut is coupled again after such rough adjustment, which can be read from the device for the visual read-out of the proportioning volume, after which a another, precise adjustment can be made, if required.
The indication devices are very expensive in this device and precise read-outs and adjustments are difficult.